In the manufacture of terry cloth, the height of the terry pile loops is a critical parameter. For example, in a typical high pile terry, approximately 55% of the total fabric is pile yarn. Any fluctuation in the pile height has an adverse effect on the fabric's weight, which may be unacceptable to customers and may require the fabric to be sold as seconds. Accordingly, uniform control of the ratio of pile-to-ground warp is a critical parameter in the manufacture of terry cloth.
Terry cloth has heretofore been produced on mechanically controlled looms manufactured by C&K Corp., Worcester, Mass.; Draper Corp., Greensboro, N.C.; and Sulzer Brothers, Ltd., Winterthur, Switzerland and others. These looms employ a "positive" pile let-off, in which a mechanical rachet device dispenses a predetermined amount of terry yarn based upon a mechanical gear ratio. Mechanically controlled looms are capable of producing terry having a consistently uniform pile-to-ground warp ratio because the correct amount of pile warp is supplied for each pick of the loom. However, such mechanically controlled terry looms typically operate at very slow rate, and require major mechanical changes to set up for a different ratio of pile-to-ground warp.
More recently, electronically controlled high speed looms, such as the Sulzer Brothers models PU and TW 11 looms, have been introduced, in an effort to make terry looms mechanically simpler and to reduce changeover time. These electronically controlled looms are described in U.S. Pat. Nos. 3,871,419 to Pfarrwaller; 4,122,873 to Pfarrwaller et al; and 4,569,373 to Vogel.
In an electronically controlled Sulzer terry loom, ground and pile warps move past a reciprocally operable reed and a displacable rocking bar. The ground warp continuously is dispensed from its supply-beam, while the pile warp is dispensed from its supply beam under the control of a "negative" pile warp let-off motor. The negative pile let-off motor controls let-off as a function of pile warp tension, with the amount of terry yarn dispensed being that amount required to maintain constant tension on the pile warp. In a typical three-pick weaving cycle, the rocking bar is maintained in a first position as the filling yarn is carried to the fell twice in succession. Before the reed is displaced a third time, the pile warp let-off motor dispenses pile yarn and the rocking bar is displaced to move the fell of the cloth towards the reed. As a result, when the reed carries the filling yarn to the fell of the cloth, loops of the pile yarn are formed in a row across the top and bottom of the base fabric. The rocking bar is then withdrawn to its initial position to permit the three-pick weaving cycle to be repeated.
Terry looms with electronically controlled motorized negative type let-off attempt to control the pile-to-ground warp yarn ratio by monitoring the tension of the pile yarn at a location near its supply beam. For example, in one version of the Sulzer machine, the ends of the pile yarn pass over a flexible beam as they are fed into the loom. A metallic flag is secured to the beam so as to move toward or away from the pile yarn supply beam as the beam flexes in response to the amount of tension applied to the pile warp. A proximity sensor is mounted adjacent the flag. This sensor produces an output voltage having a magnitude which is dependent upon the distance between it and the flag. As tension on the pile warp changes, the flag's movement alters the sensor's output voltage. This output voltage is supplied to circuitry which produces signals for increasing or decreasing the speed of the pile warp let-off motor to alter the amount of pile yarn dispensed from its supply beam and thereby maintain constant tension on the yarn. Thus, as pile warp tension increases, the pile warp let-off motor accelerates to decrease the tension, and as pile warp tension decreases, the pile warp let-off motor decelerates to increase tension.
It has been found that tension control of a negative type pile let-off produces an unacceptable variation in terry height, with a consequent unacceptable reject rate of the terry cloth. Accordingly, efforts have been made to modify the Sulzer machine control mechanism to obtain a uniform ratio of pile-to-ground warp. One such attempt is described in Dorman et al U.S. Pat. No. 4,721,134. This patent describes control of terry loop height by controlling not only the pile warp tension but also the distance the rocking bar moves during the weaving operation. Means are provided for automatically adjusting the rocking bar distances and the pile warp tension during weaving in an attempt to maintain the constant pile-to-ground warp ratio. In particular, a controller is responsive to the tension on the pile warp yarn, to the rate at which the pile warp yarn is dispensed and to a preprogrammed desired pile-to-ground warp ratio to produce a control signal for the pile let-off motor. A threshold signal is also produced when the actual pile-to-warp ratio exceeds the desired pile-to-ground warp ratio by a threshold level. The rocking bar distance is altered during weaving in response to this signal.
Unfortunately, the control technique described in the Dorman et al. patent requires a complex control system to control pile warp tension, pile let-off and rocking bar distance. Even more importantly, it has been reported that this complex control system does not always provide a consistently uniform pile-to-ground warp ratio, resulting in unacceptable terry cloth. Accordingly, the art has heretofore not provided a simple method and apparatus for effectively controlling an electronic terry loom having a negative pile let-off.